This invention relates to a mold closing unit of an injection molding machine which includes a stationary mold clamping plate and a movable mold clamping plate slidable along parallel guide rods relative to the stationary plate, movable and fixed mold dies respectively affixed to the movable and stationary plates so as to define therebetween a mold nest or cavity having a volume which is greater at the commencement of mold charging than at the termination of the mold charging. And, the mold closing unit includes a fixed, hydraulic mold closure retention cylinder, and a hydraulic traveling cylinder including a piston operable within the retention cylinder to define a pressure chamber therewith, the piston being connected to the movable die.
In known mold closing units of this general type, the volume of the mold nest or cavity is greater while the plasticized material is injected than during the subsequent pressure phase. The quality of the produced molded part is thereby considerably increased by employing this approach.
However, problems arise particularly because the specific type of enlarged volume of the mold cavity requires that the movable half of the mold die must have an exactly defined position relative to the stationary half of the mold. These difficulties have been overcome only partially but with an increasingly complex hydraulic control system. Thus, changes in length of the guide rods must be taken into consideration, for example, which are dependent upon the operating temperature, since this has an affect on the distance between the movable half of the mold in its intermediate position and the stationary half of the mold. Furthermore, a pressure is built up in the mold during injection of the plasticized material which is counteracted by means of the mold closure retention pressure which is exerted by the mold closure retention cylinder. Problems arise particularly because the viscosity of the hydraulic medium changes with changing operating temperatures which thereby influences the compressibility of the hydraulic operating medium. Moreover, it is difficult in practice to maintain the hydraulic operating medium free of air pockets, i.e. generally free of gases, which result in the formation of gas bubbles in the hydraulic operating medium during higher operating temperatures, which also exert a considerable influence on maintaining the movable half of the mold in its intermediate position. However, prior solutions to these problems require that the movable mold clamping plate be maintained in its intermediate position by the pressure of the traveling cylinder and/or the mold closure retention cylinder, although such approach has proven unsatisfactory for the reasons noted above.